From The Art and Popular Culture Encyclopedia

De rigueur is a French expression that literally means "of rigor" or "of strictness".
Something that is de rigueur is required by convention or fashion, but not by any hard rule. For example, among the upper classes during the Victorian era, wearing formal attire for dinner was de rigueur.

"De rigueur" is also a type of standard similar to a de facto standard, though de facto refers to a practice that is practically law.

Like many other foreign language phrases used in English, it is often italicized when used in a written sentence.